title: Revertible hydrogen uptake-deficient mutants of Rhizobium japonicum.
abstract: We have developed mutants of Rhizobium japonicum which are deficient in H2 uptake capacity (Hup-) and which spontaneously revert to the parent type at a frequency consistent with that of a single-point mutation (ca. 1.0 x 10(-09)). The mutagenesis by nitrous acid and the selection of the Hup- phenotype by using penicillin and chemolithotrophy as enrichment for chemolithotrophy-deficient strains are described. Two mutants retain low but reproducible levels of ribulose bisphosphate-dependent CO2 fixation when grown on a low-carbon medium under an atmosphere of 1% O2, 4% H2, 5% CO2, and 90% N2. Neither O2 nor the artificial electron acceptors phenazine methosulfate or methylene blue supported detectable H2 uptake by the free-living Hup- mutants or by their bacteroids. Plant growth experiments under bacteriologically controlled conditions were conducted to assess the mutants' performance as inocula for soybean plants. Plants inoculated with Hup- strains had lower dry weights and contained less total N than did plants inoculated with the parent Hup+ strain. Use of either the Hup- mutants or the Hup+ parent strain as inocula, however, did not significantly affect the acetylene-reducing activity or the fresh weight of nodules. These results, obtained with apparently isogenic lines of H2 uptake-deficient R. japonicum, provide strong support for a beneficial role of the H2 uptake phenotype in legume symbiosis.
mesh_terms: D002245:Carbon Dioxide / Q000378:metabolism; D006859:Hydrogen / Q000378:metabolism*; D006864:Hydrogenase; D009154:Mutation; D010088:Oxidoreductases / Q000378:metabolism; D012231:Rhizobium / Q000235:genetics / Q000378:metabolism*; D012273:Ribulose-Bisphosphate Carboxylase / Q000378:metabolism; D013025:Glycine max / Q000382:microbiology; D013559:Symbiosis
chemical_list: D002245:Carbon Dioxide; D006859:Hydrogen; D010088:Oxidoreductases; D006864:Hydrogenase; D012273:Ribulose-Bisphosphate Carboxylase